While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they often produce polymers with improved physical properties.
Since the mid-1980s, scientists have become increasingly interested in bimetallic metallocenes, and in particular, how two metal centers communicate with each other via electronic and through-space interactions (see, e.g., Reddy et al. Organometallics 8 (1989) 2107). Cooperative effects are most likely when the two metal centers are electronically coupled through a conjugated pi-electron system. Ultimately, understanding cooperative effects should let polyolefin manufacturers fine-tune polymer properties by varying catalyst structure.
While many interesting bimetallic complexes have been investigated (see, e.g., Jungling et al., J. Organometal. Chem. 460 (1993) 191; Soga et al., J. Mol. Catal. A 128 (1998) 273; and Noh et al., J. Organometal. Chem. 580 (1999), there apparently has been little or no interest in synthesizing bimetallic olefin polymerization catalysts that incorporate indigo and similar compounds (“indigoids”) as ligands.
Naturally occurring indigoid dyes have been known for thousands of years. Cloth dyed with indigotin was found in Egyptian tombs and Incan graves. Tyrian Purple, an expensive dye of the ancient world, was painstakingly isolated from mollusks of the Muricidae family. The dyes remained rare and valuable for hundreds of years. In the late nineteenth century, Nobel Prize winner Adolf von Baeyer and other chemists began elucidating dye structures and developing synthetic routes to a wide variety of water-insoluble “vat dyes,” including indigoids. By the 1920s, hundreds of indigoid dyes had been synthesized and patented. Some common examples: 
The basic indigo framework has often been elaborated by halogenation, replacement of two nitrogen atoms with sulfur (to make “thioindigoids”) replacement of one nitrogen atom with sulfur (to make “indole-naphthenes” such as Ciba Violet A), ring substitution with alkyl, alkoxy, thioalkoxy or other groups, adding benzo-fused rings, and so forth. For a few examples of indigo and indigoid preparation, see U.S. Pat. Nos. 1,133,031, 1,211,413, 1,564,218, 1,590,685, and 1,954,707.
A variety of other interesting compounds, particularly 1,3-diones, are also “indigoids” in the sense that they are isoelectronic with indigo. Like indigo, they contain a cross-conjugated, “H-chromagen” (explained below). Unlike indigo, the carbonyl groups are on the same side of the central carbon-carbon double bond. Like indigo, indigoids based on 1,3-diones have not been incorporated into bimetallic olefin polymerization catalysts.
The polyolefins industry continues to need new polymerization catalysts. Unfortunately, the organometallic complexes are becoming increasingly complicated and more expensive to manufacture. Until now, the synthesis of bimetallic complexes has involved a multistep process to produce a bridged ligand, followed by incorporation of transition metals to give the complex. The industry would benefit from a ready source of ligands suitable for making bimetallic complexes. Ideally, the catalysts would avoid the all-too-common, low-yield, multi-step syntheses from expensive, hard-to-handle starting materials and reagents.